What advantages in using progressive rendering?
Progressive render is always slower than a properly configured non-progressive render.

You use progressive rendering if you don't know how many samples to use. For example, you might use progressive on your initial render to get a feel for how many samples will look good, or to extrapolate how long a given number of samples might take. You might also use progressive if you have a specific amount of time to render, but don't know how many samples you can get done in that amount of time.

You might also use progressive if you don't know how much time you'll have to render the image. For example, if you have a SUPER complex scene and you know you're going to be out of town for a few days, you might start the progressive render, set it for umpteen-billion samples, and just let it go until you get back... whenever that is. That way, you're rendering the WHOLE time (it doesn't stop too soon with too few samples), but you also have a FULL image whenever you decide to stop (without having to wait for the last tile... or two... or more, to finish).

http://www.blenderguru.com/articles/4-easy-ways-to-speed-up-cycles/

look to 3. point

With CPU you should use a small tile size it will be faster. 

With GPU you should use a big tile size. 


Progressive rendering will make a tile as big as your image -> render time will be long

Composite could be done if you simply take a image (use openexr format if you use passes)  as input